mysql group by 用法

-- 示例表: orders
-- +----+------------+------------+--------+
-- | id | order_date | customer_id| amount |
-- +----+------------+------------+--------+
-- | 1  | 2023-01-01 | 1          | 100    |
-- | 2  | 2023-01-02 | 2          | 150    |
-- | 3  | 2023-01-01 | 1          | 200    |
-- | 4  | 2023-01-03 | 3          | 300    |
-- | 5  | 2023-01-02 | 2          | 250    |
-- +----+------------+------------+--------+

-- 按 customer_id 分组，并计算每个客户的订单总金额
SELECT customer_id, SUM(amount) AS total_amount
FROM orders
GROUP BY customer_id;

-- 结果:
-- +------------+--------------+
-- | customer_id| total_amount |
-- +------------+--------------+
-- | 1          | 300          |
-- | 2          | 400          |
-- | 3          | 300          |
-- +------------+--------------+

-- 解释:
-- GROUP BY 子句用于将具有相同值的行分组在一起。在上面的例子中，我们按 customer_id 分组，并使用 SUM 函数计算每个客户的订单总金额。